(a) Field of the Invention
This invention relates to a threaded fastener tightening method in clamping a member to be clamped to another member using threaded fasteners and a thread-clamped member which is a member to be clamped by the threaded fastener tightening method.
(b) Description of the Related Art
Threaded fasteners, such as bolts and nuts, have been widely used as means for securing parts or other purposes. For example, Unexamined Japanese Patent Publication No. 2001-153095 discloses a structure for securing a cooling fan blade to an end of a rotor shaft using a bolt. In this structure, a plain washer with sufficient rigidity and size is sandwiched between the bolt and the fan blade. Namely, in this structure, the bolt head bears on the top side of the plain washer and the back side of the plain washer bears on the fan blade.
In order to surely clamp a member to be clamped (hereinafter, referred to as a clamped member) using a bolt, a type of threaded fastener, it is necessary to give sufficient axial tension to the stem of the bolt to produce sufficient clamping force. Various methods have been proposed for controlling the axial tension acting on the bolt stem in tightening the bolt. Though the following description is given of control on the axial tension of a bolt in tightening the bolt, the same applies to control on the axial tension of, for example, a stud bolt in tightening a nut against the stud bolt.
Out of the known axial tension control methods, the simplest is a torque control method. The torque control method is a method for controlling a required torque to turn a bolt (a tightening torque). As is generally known, however, approximately 90% of the tightening torque produced in tightening a bolt is consumed by friction between internally and externally threaded surfaces and friction between bearing surfaces and only approximately 10% thereof is used to give an axial tension to the bolt. For example, in the structure disclosed in the above-mentioned Unexamined Japanese Patent Publication No. 2001-153095, almost all of the tightening torque of the bolt is consumed by friction between the bolt bearing surface and the plain washer bearing surface and friction between the internal and external threads. Therefore, it is difficult to control the axial tension acting on the bolt with high accuracy using the torque control method.
On the other hand, an angle control method is known as a method for controlling the axial tension acting on a bolt with high accuracy. This method is one for tightening a bolt using the turning angle (tightening angle) of the bolt as an indicator of the axial tension control.
In the angle control method, a particular condition in the course of bolt tightening is selected as a reference point and the bolt is tightened through a predetermined angle from the reference point. Therefore, in order to tighten a bolt by the angle control method, a reference point for measuring the tightening angle of the bolt must be selected. A known method for selecting the reference point is to select as a reference point a condition in which the tightening torque of the bolt reaches a predetermined value. Another known method is to derive an estimated bearing starting point from the gradient of a torque-angle curve showing the relation between the tightening torque and tightening angle of a bolt and select the bearing starting point as the reference point. Such a method is disclosed, for example, in Unexamined Japanese Patent Publication No. 2004-074307 and U.S. Patent Application Publication No. 2004/0027082. In the disclosed method, as shown in FIG. 9, the gradient dT/dθ of the torque-angle curve is determined in the course of tightening of the bolt and the intersection point between the line of the gradient dT/dθ and the coordinate of the tightening torque (i.e., the line of T=0) is selected as an estimated bearing starting point θ0. The bolt tightening is finished when the bolt turns through a predetermined angle θ1 from the bearing starting point θ0.
As shown in FIG. 10A, flanged hexagonal bolts in many cases have, not a plain bearing surface, but an inclined bearing surface 23. In these cases, the bearing surface 23 of the threaded fastener 20 has a conical shape radially expanding from the center to the peripheral edge. For example, when a clamped member 40 made of cast-iron is clamped with a flanged hexagonal bolt 20 made of steel, namely, when the bolt 20 has approximately the same hardness as the clamped member 40, the head 21 of the bolt 20 deforms owing to the axial tension acting on the bolt 20 so that the entire bearing surface 23 thereof comes into contact with the clamped member 40 (see FIG. 10B). Therefore, the coefficient μ of friction between the bolt 20 and the clamped member 40 becomes stabile in the course of tightening of the bolt 20 so that, as shown in FIG. 9, the gradient dT/dθ of the torque-angle curve becomes substantially constant. As a result, the bearing starting point θ0 can be accurately determined to control the axial tension on the bolt with high accuracy.